Well Erysimium Bowles Mauve does flower all year, though somewhat sparsely in the cold weather. But it gets rather too wide for pots in that position. It will need renewing every few years but cuttings strike easily. Had you thought of moving the pots to stand by the side of the step rather than on it? I can't think of anything else at all that is evergreen and flowers all spring and summer.
Do you have a back garden? If so why not get some more pots the same size and plant them up for different seasons - you can move them around when they are at their best.

If the position is shady for a good part of the day a a pair of Christmas Box plants would be nice over the winter - lovely scented flowers to greet you at the door.

Or how about helianthemums, the ones with shiny green leaves not the grey ones. They tend to be repeat flowering through the summer and are evergreen.

Look at Hebe such as
Hebe albicans spikes of white flowers in early summer.

Hebe 'Neil's Choice' violet flowers in summer and early autumn.

or
Pieris japonica (Lily of the valley bush)

Love the pots and your project. I would be tempted to put a tallish standard plant in each or something evergreen rather than flowers. On a practical note, if that is your front door those pots are liable to be knocked down when you are not looking: something I have done with mine!

I like Heuchera and their cousins......foliage and flowers and easy to look after......

I'm with Botanic - a couple of standards or a bit of topiary (if you're that way inclined). I was also concerned that they were in danger of getting in the way of the door. It would only need you to be struggling with the week's shopping and one of them would go over and get broken - much better to move them off the step - a couple of half slabs with some black pot feet would do the job.

Love the pots - I have one similar - cost me a lot of money quite a few years ago and I would hate it if it got knocked over.

Cat looks very happy catching the rays. They have an inbuilt sense of finding the best place for them that you really don't want them to have - I'll bet it's impossible to read a newspaper in your house ?

Is the area in front of the door in shade all the time, or does it get quite a lot of sun?

Sorry for delay in replying had problems getting back onto site, but up and running now I hope! Thanks so much for all suggestions. Have been looking at all the
plants suggested online. Think need to visit garden centre asap is see them all. Actually the pots are probably bigger than they appear, they are quite solid and the doorway quite wide. I took photo on a really dull day but the sun is around that way for most of the afternoon. Once planted up and established I shall post photo and let you see the winning entry! Thanks again
PS Yes right about not reading a newspaper!